Building materials such as bricks, pavers, terra-cotta tiles, or pots, hereafter referred to as "bricks," generally have a high clay content. Such building materials are not fired at extremely high temperatures like glazed ceramics. Instead, articles such as bricks are formed at temperatures sufficient to harden them for building. As a result, bricks are generally softer than ceramics or other clay products formed at high temperatures, but they also are not as brittle and therefore are deemed more suitable for general building purposes.
The use of brick as a building material dates back centuries. Because of brick's enduring qualities and limitless appearances, designers have depended on them to achieve virtually any desired style or expression. Bricks can be formed from a variety of clay-based materials and can come in a variety of colors and sizes. Clay is one of earth's most abundant mineral materials, however all clays are not suitable for brick manufacture. Clay for the production of brick must possess specific properties and characteristics in order to satisfy modern production requirements. For example, clays must have plasticity, which permits them to be shaped or molded when mixed with water; and they must have sufficient wet and air dried tensile strength to maintain their shape after forming. Also, when subjected to rising temperatures, the clay particles must fuse together. There are many variables in the manufacturing of clay and shale masonry units. Primary raw materials include surface clays, fire clays, shales or combinations of these. They must be plastic when sufficiently wetted, rigid when dried and vitrified when fired to a sufficiently high temperature. Units are formed by extrusion, molding or dry-pressing and are fired in a kiln at temperatures between 1800.degree. F. and 2100.degree. F. (980.degree. C. and 1150.degree. C.). These variables in manufacturing produce units with a wide range of colors, textures, sizes and physical properties. Naturally occurring clays are divided into specific types having particular properties. For example, clay is defined as a natural, mineral aggregate consisting essentially of hydrous aluminum silicate. It is a product of decomposition and alteration of feldspathic rocks and contains a mixture of particles of different sizes and widely differing physical, chemical and mineralogical properties. The non-plastic portion consists of altered and unaltered rock particles of which the most common and abundant substances are quartz, micas, feldspars, iron oxides, and calcium and magnesium carbonates. Organic matter usually is present in greater or lesser amounts, and frequently plays an important role in determining clay properties. The essential constituents of clays are hydrated silicates of aluminum, of which there are several, but the most important and widespread are the kaolinite group, Al.sub.2 O.sub.3 --(SiO.sub.2).sub.2 --(H.sub.2 O).sub.2 and montmorillonite group, (Mg,Ca)O--Al.sub.2 O.sub.3 --(SiO.sub.2).sub.5 The typical clay minerals--kaolinite, montmorillonite, etc., have microscopic plate-like structures which are believed to be chiefly responsible for their plasticity (formability) when wetted with water. Other important properties are: (1) hardening when dried and permanency when fired; (2) shrinkage during drying and firing; (3) the variety of colors obtainable when fired; (4) refractoriness, or resistance to softening at high temperatures; and (5) heat, sound and electrical insulation. The fineness of a clay's grain influences not only its elasticity but also such properties as drying performance, shrinkage, warping, strength and quality of marking achievable by laser energy. Clay's with high aggregate contents, sands and organic matter are prone to poor glassy vitrification by laser energy. Clays occur in three principal forms, all of which have similar chemical compositions but different physical characteristics. Surface clays may be the upthrusts of older deposits or of more recent, sedimentary formation. As the name implies, they are found near the surface of the earth. Shales are clays that have been subjected to high pressures until they have hardened almost to the form of slate. Fire clays are usually mined at deeper levels than other clays and have refractory qualities. As a rule, they contain fewer impurities than shales or surface clays and have more uniform chemical and physical properties. Clays are complex materials; surface clays and fire clays differ from shales more in physical structure than in chemical composition. Chemically, all three are compounds of silica and alumina with varying amounts of metallic oxides and other impurities.
Although technically metallic oxides are impurities, they act as fluxes, promoting fusion at lower temperatures. Metallic oxides (particularly those of iron, magnesium and calcium) influence the color of the finished fired product. The brick manufacturer minimizes variations in chemical composition and physical properties by mixing clays from different locations in the pit and from different sources. However, because clay products have a relatively low selling price, it is not economically feasible to refine clays to produce uniform raw materials. Since variations in properties of raw materials must be compensated for by varying manufacturing processes, properties of finished products from different manufacturers will also vary somewhat. The widespread usage of bricks as a building material on highly visible areas such as walkways and building fronts has led artisans to attempt to etch and decorate such materials with letters and/or graphical patterns. A common form of etching includes sandblasting the surface of the brick. Sandblasting leaves a particulate residue over the etched surface and surrounding work area. Airborne particles used in this process, such as sand and/or aluminum oxide, are known to cause irritation or carcinogenic reactions in workers and other persons passing through the area. Moreover, a mask for placement over the area to be blasted must be formed and cut. The blasted area must often be painted thereafter so that a noticeable color change will occur between the surface and the cut. Yet another method of etching includes pantograph engraving which uses a computer controlled cutting blade or bit to form channels in the surface of the object to be etched, with similar painting of the cuts thereafter.
In either instance, the surface of the brick is chipped or blasted away by the force of the sand which thereby leaves etched channels in the surface. When used to construct walkways and buildings, such channels in the bricks can collect dirt wherein the etched pattern is not readily discernable. Moreover, the sharp edges of inner "islands" of material, for instance the island formed by channeling the letter "e", are susceptible to breakage. This could render any artwork or lettering unreadable. Repair and replacement of the brick is expensive due to the way a brick is installed namely via mortar or cement attachment.
It is known that glass can be formed by melting or fusing materials under extremely high temperatures by a process called vitrification. It is also known that lasers produce intensely focused beams of light at specific wavelengths which results in localized heating of an object which falls in the path of the active beam. Laser light can be produced and amplified by a variety of sources including, for example, Nd:YAG lasers produce laser light at a principal wavelength of 1064 nanometers (nm). Nd:YAG lasers can be operated as a continuous wave (CW) laser or with pulse or frequency modification. In the latter instance, a Q-switch is used to reflect the laser beam back into the lasing chamber to build up more power before the beam is released. The result is a pulsed laser with each pulse being more intense than a continuous wave beam from the same laser unit. Other lasers, such as carbon dioxide (CO.sub.2), can be constructed and configured for different wavelengths and power outputs.
Laser marking is thereby known in the field but not for the localized vitrification of vitrescent objects such as bricks.